1. Field of the Invention
This invention relates to a layer of crystalline silicon and, more particularly, relates to a layer of crystalline silicon oriented on a sodium thallium type (dual diamond) crystalline substrate.
2. Prior Art
One of the primary limitations in reducing the cost of solar cells for terrestrial applications is the utilization of a manufacturing sequence which requires the production of highly pure semiconductor grade polycrystalline silicon, the growth of crystal silicon in bulk, typically in the form of cylindrical ingots, and the sawing of these ingots into discrete slices. By the time the actual solar cell processing occurs, i.e., the formation of a p-n junction and application of electrical contracts, the greatest portion of the product cost has been incurred. This results from the fact that the manufacturing sequence described above results in considerable material loss at the sawing stage since the kerf or "sawdust" losses can amount to about 50% of the original ingot. Also, the thickness of the finished wafer, on the order of 200.mu.m to 400.mu.m, is many times the thickness actually required to produce a satisfactory solar cell. Additionally, not only is a great amount of material wasted or unused but the process itself is costly, time consuming, requires large amounts of energy and severely separates the beginning of the materials chain from the finished product.
The nucleation of semiconductor silicon material on suitable substrate material has been the objective of numerous research efforts. See e.g., T. L. Chu, et al, "Polycrystalline Silicon Solar Cells for Terrestrial Applications", 11th Photovoltaic Specialists Conference Report 1975, p. 303. The general approach has been to find a substrate material which is inexpensive, easy to handle, does not interfere electronically with the solar cell to be formed and is susceptible to incorporation in a continuous manufacturing operation. This approach has led to the selection of non-silicon substrates and of metallurgical silicon substrates. Glass, plastic and various metals have been considered as candidate substrates. The obstacles to date to the development of an acceptable method are the quality of the crystal produced and the rate of nucleation of silicon on the substrates. Only relatively low efficiencies on the other order of a few percent have been obtained with solar cells fabricated with thin film silicon material produced on such substrates. Ideally, one would want to be able to continuously produce a layer of crystalline silicon on a suitable substrate with a sufficiently high degree of crystalline perfection to produce a solar cell with an acceptable efficiency.
It is an object, therefore,, of the present invention to provide a layer of crystalline silicon on a substrate other than single crystal silicon.
It is an additional object of this invention to provide a layer of crystalline silicon which uses a surrogate crystalline substrate that simulates the crystal structure of silicon by achieving oriented crystal overgrowth of silicon on a crystalline substrate of the sodium thallium (dual diamond) type.
It is a further object of the invention to provide a layer of crystalline silicon integral with a conductive alloy.